The invention relates to an interference screw for anchoring a tendon or ligament implant in an opening in a bone, the screw having a screw body provided with a head at one end and a penetrating end at the opposite end and said body having an outer threading.
Such interference screws are disclosed in OP Journal 14, 1998, pages 278-284 xe2x80x9cBiodegradierbare Interferenzschrauben in der Kreuzbandchirurgiexe2x80x9d A. Weiler et al., Georg Thieme Verlag Stuttgart, New York.
Interference screws have the purpose of anchoring a transplant or implant of a tendon or ligament in a bone. A channel is bored into the bone in which the transplant is placed. The interference screw is provided to be screwed into the intermediate space between the transplant and the inner wall of the channel, so that the tendon transplant is clamped between the screw and the wall. The forces acting on such a tendon or ligament, for example the cruciate ligaments in a knee joint, are considerable so that the clamping force must be correspondingly large to guarantee a durable anchor. For this purpose, the interference screw is provided with an outer threading which penetrates into the bone material on the inner side of the channel. At the same time, the outer threading engages with the transplant to be anchored.
The U.S. Pat. No. 5,383,878 discloses an outer threading formed to be a blunt, round threading, which has an approximately sinwave shaped contour about the screw axis. Interference screws having blunt threads are difficult to apply, which is a disadvantage in clinical applications.
The U.S. Pat. No. 5,688,285 discloses an interference screw whose outer threads are provided with sharp edges. The sharp edges can cut into the tendon transplant when being screwed in and weaken the transplant to the point that it releases from the anchoring position under load.
For this reason, such interference screws are mainly employed in the so-called BTB technique (bone-tendon-bone). Here, the tendon transplant is surrounded by a piece of bone in the region where it engages with the sharp threading of the inference screw, so that the interference screw penetrates the end of the tendon surrounded by the bone material on the one hand and the bone material at the inner wall of the channel on the other hand.
An object of the present invention is to provide an interference screw which allows a simple and secure anchoring of a tendon or ligament transplant.
According to the present invention, the object is achieved in that the outer threading of the screw body is formed to be sharp in an area adjacent to the penetrating end and is formed with a blunt threading in the following regions.
When applying the interference screw, the sharp threads adjacent the penetrating end clearly define an entrance path for the following blunt sections of the threading. Approximately complementary screw line paths are cut into the inner side of the channel. The blunt regions of the threading can then be simply screwed into these cut lines, where the blunt regions of the threading then provide sufficient compression to anchor the tendon transplant to the inner side of the channel. In this manner, the interference screw can be simply mounted and is properly guided. A further advantage is that it is ensured that the transplant tissue is not impaired, i.e. not severed or separated in the region of the blunt threading.
In a further embodiment of the present invention, at least one thread winding is formed to be sharp. By providing at least one winding of the threading to be a sharp, cutting thread it is guaranteed that a guide line or guide path is cut about the entire circumference of the channel wall, in which the subsequent blunt regions of the threading can exactly follow.
In at least one embodiment of the present invention, the screw body is tapered at the penetrating end and the sharp threads extend to about the position of maximum outer diameter of the outer threading.
This feature has the advantage that the tapered end of the interference screw can be placed in the free space between the end of the tendon and the channel and can be correctly aligned for application. Since the tendon material has a certain compressibility, the material in the region of the threads at the penetration end can be displaced to the side, so that the danger of impairment by the threading is further reduced. Corrections in which the screw is removed and then screwed in again are thus no longer necessary. It is also ensured that the sharp threads extending up to the maximum core diameter are able to cut the entry path in the inner wall of the bone channel. The handling is thus further simplified.
In a further preferred embodiment of the present invention, the transition from sharp to blunt threads is gradual. The feature has the advantage that the guidance of the following blunt threads into the spiral paths previously cut by the sharp threads in the inner wall of the channel is very smooth.
In a further embodiment of the present invention, the outer threading is formed to be buttress threads. The feature has the advantage that the forward flanks of the tooth profile can penetrate into the material rather softly. With the relatively compressible tendon material, it is correspondingly radially shifted or compressed without negative effects on the transplant. Both the bone material and the tendon material have a certain resiliency, so that the material is pushed back to the region of the back flanks and forms a mechanically rigid, closed connection with the interference screw.
In a further embodiment of the present invention, the interference screw is made of biodegradable material. This feature has the advantage that with time the bone material grows into the space previously occupied by the screw material. After time, this ensures a secure seating of the tendon transplant, because an organic growth with the tendon transplant takes place. A loosening between the interference screw and the anchored tendon transplant due to loads or degenerative deformations is therefore excluded and a durable secure seating is guaranteed. This also opens the possibility of providing new borings, if revision is necessary, to anchor another or different transplant at the same bone region, since bone material has again accumulated through subsequent growth which can be bored again.
In a further embodiment of the present invention, the screw body is formed as a hollow body and in a further embodiment is preferably provided with several perforations.
This feature has the advantage that the formation of a three-dimensional bone structure in the region of the screw is further promoted by the perforations. Bone material can grow into the interior of the hollow body through the perforations or can grow together and form an intimate connection with bone material already present in the interior space of the hollow body, whether this is bone slurry or pieces of bone. This process can run parallel to the biological degradation of the material of the screw, so that a secure seating of the transplant is always ensured and with time, more and more natural bone material is formed in the region of the transplant. The form, distribution and the number of perforations are selected such that sufficient stability of the hollow body is retained, even though numerous openings for growth are present.
In a further embodiment of the present invention, the perforations are provided between the windings of the threads. The feature has the advantage that the windings can retain a continuous spiral contour, which simplifies a smooth screw mounting of the interference screw. The perforations for bone tissue growth are then present in the intermediate spaces in the actual screw body.
It will be understood that the above-mentioned features and those to be discussed below are applicable not only in the given combinations, but may be present in other combinations or be taken alone without departing from the scope of the present invention.